Noise suppressor circuits



June 3, 194 1. F T, BUDELMAN 2,244,639

NOISE SUPPRESSOR CIRCUITS Filed March 14, 1939 ATTORNEY Patented June 3, 1941 UNETE .TES

NO! SE SUPPRE S SOR- CIRCUITS Frederick T. Budelman, Little Falls, N. J assignor to Fred M. Link, New York, N. Y.

3 Claims.

This invention relates to radio receivers and more particularly to short-wave superheterodyne receivers employing a high intermediate frequency.,

The main objects of the invention are to suppress noise in radio receivers and to improve the signal-to-noise ratio therein for both strong and weak signals.

Another object is to provide an improved combination of noise suppressor and squelch circuits in a radio receiver.

A specific object is to provide a receiver which is effective against ignition noise and the like, for mobile use; for instance, in policepatrol cars for receiving radio signals of the order of 35 megacycles.

Other objects will become apparent as the description proceeds in connection with the accompanying drawing, in which:

Fig. 1 is a, circuit diagram illustrating the main features of the invention; and

Fig. 2 is a portion of the circuit diagram of a receiver employing the invention.

Like reference characters indicate similar parts in both figures.

The most troublesome noise encountered in the use of radio receivers, especially on short waves, is caused by electric discharges such as are produced by A. C. power leaks, switches, keying of telegraph transmitters, defective neon signs, static electricity, and especially ignition systems of automobiles. Such noise is sometimes called the pulse or shot type, because its radio wave has a steep front of high peak amplitude and is highly damped. Its fundamental frequency may vary from individual clicks up to several hundred impulses per second. However, each impulse is of short duration relative to the time between successive impulses. The present invention employs this characteristic to distinguish between noise and signal so that the noise is suppressed in the receiver but the signal passes through substantially undisturbed. Preferably the noise suppressor is included in a detector circuit and is followed by a squelch circuit associated with the first audio frequency amplifier. The squelch circuit blocks the audio amplifier except when radio signals are being detected.

Referring to the drawing in detail, Fig. 1 illustrates adetector circuit, noise suppressor, squelch circuit, and first audio frequency amplifier for a radio receiver in accordance with the invention. A, transformer 5 having a secondary 6 tuned by variable condenser 1' provides the input to the detector circuit; Transformerb may be the last intermediate transformer in a superheterodyne and maybe preceded by a suitable antenna, first detector, and other intermediate frequency circuits (not shown). The current from secondary 6 is rectified by a diode detector tube 8 and impressed upon a load resistor H] which is icy-passed for the rectified frequency by'condenser H. The cathode of tube 8 and the positive end of resistor l0 are grounded at l2.

The noise suppressor comprises a condenser it and a diode rectifier l5 connected in series across resistor it, the cathode of tube l5 being connected to the negative end of the resistor and its anode to condenser it. A leak resistance i6 is shunted across the anode and cathode of tube The audio frequency amplifier starts from the negative end of resistor in which is connected through a coupling condenser ll to the control grid it of an audio frequency amplifier tube 26; The cathode of tube 23 is icy-passed. to ground through a high capacity condenser 2i and is conductively connected through a bias resistor 22 to a point 24 between voltage-dividing resistances 25, 26 whereby point Z l is maintained about twenty volts positive to ground A positive plate supply 2'! is connected to one on dof resistance 23 and also supplies plate current to tube 28 through a load resistor 28. The audio frequency potentials developed across resistor 28 are .passed through a coupling condenser 39 to a power amplifier and reproducer (not shown).

The squelch circuit comprises an adjustable tap ti on resistor it" connected through a high resistance 32 to the control grid 34 of a squelch tube 35, the cathode of which is grounded at 3B. The grid and cathode of tube 35 are by-passed for audio frequency by a. condenser 37. The plate of tube 35'i's supplied with positive potential from point 2 1 through a resistor 38 and is also connected through a high resistance it to grid 18 of audio tube it. A condenser ii by-passes the plate and cathode of tube 35. for audio frequency. A switch 52 may be provided to disconnect the plate of the squelch tube and thereby render it inoperative when desired. The potential across resistor iii may be tapped for. automatic volume control as indicated at 43.

The operation of the arrangement shown in Fig. l is as follows:

The desired input frequency from tuned circuit 8, "i is rectified in known manner by diode 8, the

modulations carried thereby appearing across" resistor l6 as audio frequency potentials superposed on the D. C. component of the rectified frequency. The capacity of condenser H is large enough to make the D. 0. component nearly equal to the peak voltage of the rectified frequency, but small enough to offer high reactance to the audio frequency modulations. The D. C. component also charges condenser i i through diode l which is conductive in the proper direction for charging the condenser thereby. However, condenser M has a capacity such that many cycles of input frequency are required to build up its charge to the rectified potential. The audio frequency modulations further build up the charge in condenser i 4 to their peak potential and this charge is retained therein because the condenser cannot discharge through tube l5 during the opposite half-cycles of the audio frequency. The charge thus built up in. condenser I 4 by both the rectified input and its modulations can only leak off through resistance it. The values of condenser I l and resistance it are selected to give a time constant which is long relative to the cyclic periods of the input frequency and its audio frequency modulations. For instance, condenser M may be .05 microfarad and resistance It may be 1 megohm, thus giving a time constant of .05 second whereby the con denser charge is substantially retained from peak to peak of the audio frequency.

With the desired signals being detected by tube 8 and with condenser 54 charged as stated, if a series of noise impulses (as above described) arrive at the detector input, they will be rectified by tube 8, but each impulse is of such short duration that it alone can not materially alter the charge on condenser it. The low impedance offered by this large condenser permits the steep wave front of the impulse to pass between the plate and cathode of tube i5 whereby it is bypassed around the resistor Hl. Hence the impulse is effectively suppressed. During the relatively long interval between one such impulse and the next, any slight increase in the potential of condenser M has time to leak through resistance 56 so that said next impulse also will be by-passed through tube without building up a charge on condenser M such as is built up by the continuous rectified input and its audio frequency modulations. In this manner, the noise impulses are prevented from interfering with the detected signal modulations, which latter alone are passed on to the audio frequency amplifier.

The operation of such detector and noise suppressor circuits is independent of the strength or weakness of the received radio signals or of I the percentage modulation because the combination of condenser i4, tube 15 and resistance It tends to prevent the noise impulses from changing whatever potentials are developed across resistance it by the rectified frequency and its modulations. Even'when there are no signals present, the noise suppressor acts to by-pass noise impulses so that they are not heard in the audio frequency output.

Proceeding now with the operation of the audio amplifier and squelch circuits, the audio frequency modulations from resistance to are impressed through coupling condenser E 7 upon grid [8 of the first audio tube 29 wherein they are amplified in the usual manner and then coupled to a power amplifier stage (not shown) by plate resistance 22 and condenser 39. When no input frequency is being rectified in tube 8, the grid 3 in squelch tube 35 is substantially at ground potential by reason of its conductive connection through resistance 32, tap 3!, and resistor [It to of any signals or noise whatsoever.

ground l2. The cathode of tube 35 also being connected to ground at 36, gives tube 35 a zero grid bias. The tube 35 will therefore draw plate current from point 24 through resistance 38, provided switch 42 is closed. By reason of the connection of the plate of tube 35 through resistance 40 to grid 58 of audio tube 28, said grid will be made negative with respect to its cathode by the amount of plate current drop in resistance 38. This negative potential on grid l8 blocks tube 20 and substantially prevents reproduction However, when radio signals are received and rectified, a desired portion of the D. 0. component across resistor i0 is tapped at 3| and impressed upon control grid 34 of tube 35. Grid 34 is thus made negative with respect to its grounded cathode and reduces the plate current substantially to zero. This action of tube 35 removes the potential drop in plate resistance 38 and thus removes the negative blocking potential from the grid l8, thereby permitting tube 25 to open up and amplify the audio frequency. Tube 20 obtains its normal operating bias from the plate current drop through resistance 22 in its cathode lead. Thus tube 35 permits amplification in tube 2!] only when signals are being received and detected.

It is a feature of the present invention that the squelch circuit just described follows the noise suppressor I 5, l5, Hi. If a squelch circuit precedes a noise suppressor circuit as has heretofore been proposed, the noise impulses cause the squelch circuit to open up the amplifier and thereby cause the noise to be heard as intermittent pulses timed by the rate at which the squelch circuit can open and close the amplifier. An advantage of the arrangement disclosed herein is that noise impulses are by-passed as explained above and thus are unable to open up the squelch circuit.

The circuit shown in Fig. 2 of the accompanying drawing contains substantially the same elements and operates in the same manner as the circuit of Fig. 1 above described. Fig. 2 is part of a circuit employed in a commercial receiver wherein the detector and noise suppressor diodes are combined in a single tube 45 which may be a type 6H6 twin diode. Similarly the tube elements for the first audio amplifier and the squelch circuit are combined in a single tube 45 which may be a type 608G twin triode amplifier. The detector input transformer 5 is shown in Fig. 2 with a tuning condenser 4'! in parallel with the transformer primary, said condenser and primary forming the output circuit of the last intermediate frequency stage of the receiver. The other circuit elements shown in Fig. 2 are numbered to correspond with similar elements in Fig. 1 and have the same function.

The circuits above described have been found to be especially suitable for reception in the police band of 30 to 42 megacycles with .an intermediate frequency of 5 mega-cycles. On such short waves igniton noise is very troublesome, especially due to the operation of police cars in the midst of trafii'c and on streets shielded by buildings. Radio signals having a strength of only 1 microvolt per meter can be received successfully through serious ignition interference by means of the above circuits. It is obvious, that the invention is not limited to any particular band of frequencies or type of receiver, but is generally useful in radio reception.

Although a preferred form of the invention has been disclosed, it will be appreciated that various changes and equivalent forms can be made Without departing from the spirit or scope of the invention as defined in the appended claims.

What I claim as new and original and desire to secure by Letters Patent of the United States is:

1. In a radio receiver, the combination of a detector circuit, a noise suppressor associated therewith and adapted to exclude noise impulses from the output of said detector circuit independently of the amplitude or percentage modulation of received signals, and audio frequency amplifier coupled to the output of said detector circuit, and a squelch circuit connected to said amplifier whereby said noise suppressor prevents operation of said squelch circuit by impulse type noise in the absence of high frequency signals in said detector circuit.

2. The combination defined by claim 1 in which said detector circuit has an output resistor, and said noise suppressor comprises a condenser in series with a parallel-connected diode rectifier and leak resistance, said series-parallel combination being connected across said output resistor.

3. In a radio receiver, the combination of, a detector circuit, a noise suppressor circuit adapted to exclude noise impulses from the output of said detector circuit, an audio amplifier following said detector circuit, and a squelch circuit which normally blocks said audio amplifier and, in combination with the said noise suppressor circuit, permits amplification of detected signals only.

FREDERICK T. BUDELMAN. 

